A radio transceiver has an ability to both receive and transmit information modulated on a carrier frequency. In general, to provide satisfactory performance, the carrier frequency must be predetermined. In many radios, the carrier frequency used for transmission may be identical to, or may otherwise be a function of, the carrier frequency used for reception.
During reception, a radio can ordinarily compensate for various internal frequency errors (caused, for instance, by component tolerances, age, or temperature) by use of various prior art frequency steering or locking techniques. In general, a receiving radio utilizes a steering mechanism, such as an automatic frequency control (AFC) loop, to locate the general vicinity of the carrier frequency it seeks to receive or a locking mechanism, such as a phase locked loop, to precisely locate the reception frequency.
During transmission, however, different circumstances prevail. The carrier frequency transmitted by radio must be as precise as possible. In a base station radio, this can be accomplished in many ways, including the use of high tolerance (and expensive) components, redundant (and expensive) calibrating and monitoring circuits, and the like. In a mobile radio transceiver, however, where space and cost parameters are very important to the end user, such remedies are often not available. Therefore, the problems of maintaining a correct transmission frequency are greater.
In wideband systems, generally available components can be, and are, used in today's mobile radios with reasonable success with regards to accuracy of transmission frequency. Though not as accurate as a base station transmission, the width of the channel spacing provided enough latitude to ensure reasonable performance. However, in narrowband systems, the importance of locating the reception frequency and maintaining the correct transmission frequency has increased.
Therefore, there exists a need in narrowband systems for a better means of locating reception frequencies and providing accurate transmission frequencies in a mobile radio. The desired mechanism should not contribute unduly to the expense of the radio nor its complexity of operation. Further, the desired mechanism should not greatly impair the manufacturing process or serviceability of the radio.